Résumé :
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Communication n° 639. PKB/Akt kinases are involved in signaling downstream of growth factor receptor tyrosine kinases and phosphatidylinositol 3-kinase. Akt1 and Akt2 are the two principle Akt isoforms expressed in all tissues. We have used small interfering RNA (siRNA) to analyze the specific function of each PKB/Akt isoform in mouse C2 myoblasts. When proliferative C2.7.4 myoblasts were microinjected with Akt1 siRNA, less than 10 % injected cells were still proliferating when compared to controls. In contrast, siRNA to Akt2 had no such effect and instead resulted in slightly higher growth rate. This was confirmed by analyzing cyclin A and PCNA expression in cells transfected with siRNA to Akt1 or Akt2. In differentiating myoblasts, whereas knockdown of Akt1 had no effect on the early differentiation marker myogenin and confirmed the requirement for Akt1 in cell survival, siRNA to Akt2 strongly reduced myogenin expression (over 75 %) and extended cyclin A expression. A specific rescue of the inhibitory effect of siRNA injection could be obtained by microinjection of the corresponding purified kinase isoform into siRNA injected cells: active Akt1 restored expression of cyclin A and cell cycle progression, an effect not observed with Akt2 kinase. Taken together, these data show that PKB-alpha/Akt1 and PKB-beta/Akt2 isoforms perform opposite roles in myoblast proliferation : whereas Akt1 is required for cell cycle progression, Akt2 appears to participate in cell cycle inhibition in addition to being specifically required for myogenic differentiation as we showed before.
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